This invention relates generally to enhancement of semiconductor device contact pad surfaces for reliability improvement and other purposes and, more particularly, to repairing probe-damaged pads, reinforcing semiconductor pads in general, enlarging semiconductor device pads for various purposes, and buffering semiconductor device pads. The invention is applicable to any semiconductor device with contact pads on a surface thereof, such as, but not limited to, entire semiconductor wafers, including wafer scale integration (WSI) devices, individual semiconductor dice, and High Density Interconnect (HDI) multi-chip semiconductor device modules, such as disclosed in Eichelberger et al. U.S. Pat. No. 4,783,695, and related patents such as are identified hereinbelow.
Semiconductor devices, such as integrated circuit chips, typically have contact pads on a major surface. In conventional packaging technology, electrical connections to these contact pads are made by ultrasonic wire bonding. As part of the device manufacturing process, the contact pads are probed for electrical testing. In some situations, semiconductor device pads are probed a number of times, for various desired parameters. Such electrical probing results in varying degrees of probe damage to the pads, and in some cases multiple areas of damage to a single pad occur. The probing can leave areas of a pad literally detached from the underlying substrate.
In addition to probe damage, the various processing steps performed in pad etching can cause voids in the metal pads if not done carefully, leading to weakened areas.
One particular reliability problem occurs where forming of a via in an HDI module by laser dithering, such as described in Eichelberger et al. U.S. Pat. No. 4,894,115, may actually cause a local meltback of the pad, lifting a portion of the pad off of the semiconductor material. The lifted portion of the pad thus has a severely reduced conductive path for dissipating the laser energy heat generated during the laser drilling process. If processing were to continue, it would likely result in an electrical open circuit, or discontinuity, between the subsequently-applied via metallization and the pad. Although this condition is detectable by visual inspection, its repair is costly, as the standard repair procedure involves total removal of the adhesive overlying the pad and the dielectric film atop the adhesive, both of which are employed in typical HDI modules, along with replacement of the damaged semiconductor device.
Another particular reliability problem may occur when the HDI assembly process reaches the point of via metallization in ohmic contact with what remains of the contact pad. However, due to the thinned area of the pad which the via metallization actually contacts, current density could be many times the theoretically allowed limits, causing a later electrical open circuit or discontinuity as a result of a slow electromigration or a direct burnout under surge conditions. Such failures are more likely in high radiation environments.
Temperature constraints are typically placed on processes for repair of such damaged pads. Metal application methods, such as those used in large scale integration (LSI) or very large scale integration (VLSI) are not directly applicable. Generally, a post anneal at a temperature of approximately 400.degree. C. is necessary to achieve ohmic contact and low resistivity. This temperature is objectionable for at least two reasons. First, in the event the semiconductor device is included in an HDI circuit module in the process of being assembled, such high temperature would destroy the polymer layers. Second, the fact that the damaged areas of the pads are probe damaged implies that the semiconductor device has passed its functional and parametric tests. Reheating the semiconductor device to such high temperature would necessitate retesting.
While the discussion up to this point has been primarily in the context of contact pad repair, which is one form of what is herein termed "enhancement", there are a variety of situations where it may be desired to enhance a semiconductor device contact pad in some other manner. For example, it may be desired to generally reinforce a contact pad, even in the absence of known damage. It may be desired to enlarge a contact pad, such as for the purpose of facilitating test probing. It may be desired to add a buffer pad for connection to a temporary wire bond for testing purposes and that may be pulled off when testing has been completed. Such operations have heretofore usually resulted in irreparably damaging a semiconductor device pad.